This invention relates generally to air pollution monitoring systems and more particularly to an automated system for continuous electrochemical monitoring of air pollutants to obtain quantitative measure thereof and maintain permanent record.
As legislation, Such as the Clean Air Act of 1970, is increasingly enacted to define permissible concentrations of pollutant emissions to establish acceptable levels of air pollutants for cities, states, and regions, it becomes increasingly more important to find and develop analytical procedures that are both sensitive and specific for these substances. Then, when more accurate monitoring techniques are made available, standards can be set to limit concentration levels of pollutants in the atmosphere.
In the art, the most widely used instruments for measuring pollutant concentrations in the atmosphere are based upon wet chemical techniques wherein coulometric, colorimetric, or conductometric procedures are utilized. In the coulometric procedure, for example, the pollutant is passed through a collection fluid and a measurement is then made which can be compared with a prior reference measurement. In the standard reference method for nitrogen dioxide, for example, the gaseous sample is bubbled through a solution of sodium hydroxide, where the NO.sub.2 is converted to stable sodium nitrate. After collection, the nitrate is diazotized with phosphoric acid, sulfanilamide and coupled to N-1-naphthylethylenediamine; the concentration of the resulting chromophore is then determined colorimetrically.
Recently, monitors based upon such chemical techniques as electrochemical transducers have found increased use by environmental scientists in pollutant detection. These devices depend on selective electrocatalytic oxidation (or reduction) of the pollutants in the cell. In a typical transducer, the sample gas flows past a semipermeable membrane that allows a small, constant fraction to enter the cell. Inside the cell, the sample diffuses through a thin liquid film to an electrocatalytic electrode, where a charge transfer reaction occurs. Bulk electrolyte and a reference electrode make up the rest of the cell. In operation, the cell produces an electrical potential that is directly proportional to the concentration of pollutant in the sample stream and, within limits, is independent of the sample flow rate. The problems involved in determining chlorine and chloride in air with these prior art methods are (1) plugging of the membrane by particulate matter, (2) low collection efficiency which results in poor precision, (3) frequent maintenance schedules are required, and (4) slow response to changes in pollutant levels.